1. Field of the Invention
The invention relates to apparatus, and an accompanying method for use therein, for a thermodynamic material testing system that is capable of controllably inducing very large strains in metallic structures, specifically in crystalline metallic specimens. Additionally, the system can also simultaneously direct resistance heat or conductively cool such a specimen then under test, under controlled conditions, in order to establish isothermal planes at a desired substantially uniform temperature throughout a work zone in the specimen. The invention is particularly, though not exclusively, suited for simulating the performance of high-speed multi-stand rolling mills and in properly configuring those mills to produce metallic material with very fine-grained crystalline structures.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products.
Such materials, with relatively few exceptions, solidify in ordered structures that have atoms arranged in a pattern that repeats itself periodically in three dimensions. Whenever an ordered crystalline structure that forms an entire piece of solid material has a single orientation, the material is viewed as being a single crystal. Polycrystalline aggregates, which are formed of assemblages of large numbers of relatively small crystals, each being a so-called xe2x80x9cgrainxe2x80x9d, are the most common form of metals. Within a pure metallic material, each grain has the same composition and structure as that of all of its neighbors, but differs from them in size, shape and orientation. Orientation differences result in the appearance of definite grain boundaries at interfaces between adjacent crystals. These differences significantly affect the properties of the material and to a great extent more so than do the grains themselves.
One important property of a metallic material is its material strength. Commercially speaking, materials with relatively high strengths are extremely. important. A predominant reason is simply that an item can be manufactured to contain less material and hence with generally less weight, if a higher rather than a lower strength material is used. Based on the relative material strength, the weight savings can be appreciable. Oftentimes and not surprisingly, with currently available materials, these weight advantages may well be offset, in certain applications, by increased material cost.
As the grain size of a metallic material decreases, the strength of that material increases. Inclusion of high angle grain boundaries into the material further enhances material strength. Given this, for many decades, considerable effort has been expended in the art to devise and implement techniques for reducing grain sizes, and particularly those techniques that can be used with relatively low cost materials. Clearly, those techniques that readily lend themselves to use in mass production are in greatest demand and hence very eagerly sought by material manufacturers.
Currently, metallic materials are typically fabricated through rolling, forging or extruding-based techniques into sheet, strip or wire, i.e., intermediate products, which are thereafter appropriately and ultimately formed into a shape of a final product. Each of these techniques is governed by interaction of a number of different process parameters all of which significantly influence physical characteristics, such as grain size and shape, and grain boundary orientation, of the final material produced thereby.
Compressively deforming a polycrystalline metal causes its grains to distort. For example, when a metal is rolled, its grains are elongated in a direction of material travel through a rolling mill and cross-wise of longitudinal axes of the rolls, i.e., perpendicular to a roll-metal interface. When a short cylindrical piece of material is compressively deformed, such as through forging, in a direction parallel to a longitudinal cylindrical axis of the material, the length of the cylinder decreases while its diameter increases as the material flows radially outward while it is being deformed; hence, its grains flow elongate here too. Such mechanical working of a metal occurring below its recrystallization temperature, typically referred to as xe2x80x9ccold/warmxe2x80x9d working, increases internal energy within the crystalline structure. New grains can be formed when additional energy, typically through post-deformation thermal treatment though occurring below the recrystallization temperature, is imparted to the crystalline structure of the material.
The volume of a metallic material at a constant temperature and pressure is itself basically constant. Hence, with the exception of heavily distorted crystalline structures, deforming such a material to change its shape will not appreciably change its volume. When such a material, in the form of a sheet or ingot, is deformed in a rolling millxe2x80x94i.e., when a roll stand imparts sufficient pressure to the material in excess of the material yield strength, the resulting material will be reduced in thickness but its length will increase. The material will continue to flow elongate as its thickness is increasingly reduced through successive rolling operations.
Grain size and material strength are directly determined through the amount of strain imparted to a metallic material. Deformation causes strain. As the strain increasesxe2x80x94up to a point where recrystallization occurs, the microstructure in the material moves, grain refinement occurs and concomitantly grain size decreases. Owing to reduced grain size, the strength of the material increases. Currently, most production rolled steel has a grain size in the 10-50 micron range. If material grain size can be reduced to approximately 1 micron from, e.g., 10 microns, then strength of the resulting material will likely double, if not increase further. Obviously, producing material with such fine grains and high material strengths has profound commercial importance.
In that regard, if lighter materials, such as aluminum, can be processed to yield very small grains and hence significantly enhanced material strengths, then such materials could be used to fabricate parts heretofore formed of heavier materials, such as steel alloys, that possess the same strength. Alternatively, in other applications where material weight may not be an important factor but material cost certainly is, then if relatively low cost materials, such as common low cost steel alloys, could be processed in a fashion that would yield smaller grain sizes and hence increased material strength than heretofore realized in the art, then substantial cost savings will likely result if these materials could replace relatively high-cost high-strength materials.
Another technique commonly used to increase material strength in structural steels has been to alloy various expensive elements into these steels. However, the resulting alloys still tend to be extremely costly and hence economically unsuitable for use in many applications. If the grain size of low-cost steels could be reduced to the point at which such steels possess material strength comparable to that of such alloys, these steels could form a rather cost-effective alternative to such alloys.
Hence, a continual effort has occurred in the art over the course of many years for techniques that can significantly reduce material grain size in order to yield relatively low cost materials that have increased strength.
Mathematically, true strain (xcex5), for a compressive deformation of a specimen, is defined as xe2x88x92ln(h0/h) where h is final specimen height and ho is initial specimen height, and true strain rate is dxcex5/dt or xe2x88x92(1/h)(dh/dt). Strain is linearly cumulative, provided the specimen material does not recrystallize during an entire deformation process. In that regard, if xcex5i represents an amount of strain resulting from deformation i to a specimen, then total strain imparted to that specimen, xcex5t, is simply a sum of the individuals strains produced by all the deformations (n), as given by equation (1) as follows:                               ϵ          t                =                              ∑                          i              =              1                        n                    ⁢                      ϵ            i                                              (        1        )            
Various conventional approaches are taught in the art to reduce grain size. These include, e.g., reduction of metallic material to a very fine powder with subsequent compaction of the powder into a shape of a desired part with or without sintering (xe2x80x9cpowder metallurgyxe2x80x9d based approaches); freezing liquid metallic material by spraying it onto a very cold wheel, typically fabricated of copper, in a fine stream to induce a very high cooling rate; and casting large billets (ingots) or slabs of metallic material which are subsequently reduced to much thinner material by mechanical, i.e., compressive, work to strain the material. Mechanically working such billets or slabs is most commonly used technique where large quantities of material, such as millions of tons of steel or aluminum, need to be formed.
Unfortunately, the capital costs associated with establishing a new production mill, such as a multi-stand rolling mill, for producing a desired intermediate product are staggering in and of themselves. For that reason, relatively few new mills, if any, are currently being built with a marked preference existing in industry to employ appropriate and existing facilities that have requisite idle capacity. However, even with existing facilities, additional sums will often still need to be expended, typically through experimental trials on the mill and using actual material, to properly set the process parameters such that resulting material produced by the mill will possess desired physical characteristics. Given the cost of mill downtime (non-production time), these additional costs can become quite significant in their own right. Hence, a mill owner, seeking as large a return on invested capital as possible, will seek to minimize mill downtime to the extent possible.
Consequently, in an effort to drastically reduce the costs associated with properly setting a production mill or even using it for experimental purposes, considerable effort has occurred in the art, again over the past few decades, and is still occurring, to accurately simulate rolling, forging and extruding techniques, in a laboratory environment, on small metallic specimens. Such simulations, if properly conducted and particularly using proper schedules of material deformation(s) and temperature treatment(s), should yield results that will very accurately, when scaled upward, replicate material behavior under actual production conditions. Fortunately, a rather large number of simulations can be run at an aggregate cost that is typically rather negligible when compared to the cost of even a few hours of lost production from mill downtime; thus, providing a mill owner with an effective experimental avenue for use in determining proper mill settings to yield a desired material but without incurring excessive costs to do so.
While simulations using conventionally available thermodynamic material testing systems, such as the xe2x80x9cGLEEBLExe2x80x9d systems manufactured by the present assignee (which also owns the registered trademark xe2x80x9cGLEEBLExe2x80x9d), advantageously provide substantial cost and time savings and, for a wide variety of deformation-based processes, yield rather accurate results, these simulations are simply incapable of imparting sufficiently high strain to a specimen that would yield very fine grain sizes.
In particular, currently available material testing systems, such as the xe2x80x9cGLEEBLExe2x80x9d systems, can controllably deform a bar specimen along two specimen axes by alternately compressing the specimen at or near its midspan and along axes that are perpendicular to the bar and to each other with a compression being applied along one such axis at a time. If the bar is to remain straight, equal compressive forces are simultaneously applied to opposite sides of the bar. If a small portion of the bar length is compressed near the middle of the bar, typically in a so-called xe2x80x9cwork zonexe2x80x9d, as is conventionally done, then, during each compression, specimen material flows from a compressed work zone region towards the ends of the bar causing the bar to elongate. As the bar is alternately compressed along these axes, the volume of material in the work zone will continue to shrink, as the material there is successively compressed, and the bar will continue to elongate. However, such repeated compressions are problematic. In that regard, if a fairly large bar specimen having a square cross-section of 900 mm2 (30 mm-by-30 mm) is compressed along two axes (perpendicular to the major axis of the specimen) to a strain of 4.6, then the resulting bar cross-section will be only 9 mm2 (3 mm-by-3 mm). This cross-section is simply too small to support subsequent test work on the specimen, i.e., successive deformations, as well as machining the specimen to a size needed for further analysis.
Further, even with using lessened amounts of strain, specimen area in the work zone after one or more deformations tends to be relatively small compared to the surface area of the compression face of the anvils. Hence, thermal control of the specimen can be compromised as a result of heat loss occurring through the anvils during each compression. Moreover, while highly uniform micro-structures can be produced using conventional xe2x80x9cGLEEBLExe2x80x9d systems, the resulting specimen is often too deformed to be easily held during subsequent machining of its work zone down to a size sufficient for use in subsequent analysis. As such, suitable specimen ends must be added, once all the deformations have been completed, on one axis of the specimen by either welding or bonding to facilitate such machining. Unfortunately, the process used to add these ends often, particularly if it involves welding, corrupts the specimen microstructure produced by the deformations.
Furthermore, if a cylindrical specimen, rather than a bar, were compressively deformed in a longitudinal direction between two compression anvils, then specimen material remaining in the work zone would distort so appreciably when large strains are imparted that the resulting deformed specimen would also no longer be suitable for further deformation or machining. Illustratively, if a strain of 3 is imparted to a specimen having a diameter of 10 mm and a height of 10 mm, the resulting specimen will be compressed to a height of 0.499 mm but with an increased diameter, due to radial flow elongation, of 44.82 mm. Owing to increased surface friction between the material and each anvil, such a large surface area coupled with a thin thickness makes further reductions impractical. Further, the compression anvil would need to have a diameter of at least 50 mm. Consequently, imparting a strain of 10 to such a specimen could not be readily accomplished, if at all. Moreover, should the specimen area perpendicular to an axis of compression exceed a surface area of a compression face of the anvil, then, during any one compression that were to impart a strain over 3, the specimen material, due to flow elongation, would flow beyond the anvil face and hence no longer be compressed. This, in turn, precludes the specimen from being compressed any further; thus, effectively preventing further grain refinement.
Consequently, to yield a specimen that, after being deformed in conventional thermodynamic testing systems, still presents an adequately sized cross-section in view of flow elongation, the total amount of strain that can be induced is typically limited to 2 or less. This amount of strain is simply far less than that needed to cause necessary grain refinement to appreciably reduce the grain size down from a range of 10-50 microns.
The art teaches two other techniques of inducing high strain in a metallic specimen. These techniques involve: (a) rigidly securing each end of a cylindrical bar in a clamp and twisting the bar to introduce torsional deformation to a work-zone, and (b) deforming a cylindrical specimen, by extrusion, through an equal channel angular (ECA) pressing process. However, both of these techniques have proven to be quite problematic in practice and hence rather deficient in accurately simulating production techniques.
Specifically, while a specimen is subjected to torsional deformation, specimen material in the work zone flows in shear planes, with the amount of flow dependent upon an amount of shear. For any point in the work zone, the amount of shear to which material at that point is subjected depends upon a radial distance of that point from an axis about which the specimen ends are twisted, here the longitudinal axis; with torsion being zero along the axis and maximum along a periphery of the specimen. Unfortunately, since the amount of shear is radially dependent, non-uniform deformation results which, in turn, produces non-uniformities in the specimen grain structure; thereby, from a practical standpoint, yielding generally worthless results.
In an ECA pressing process, a billet specimen is pressed, i.e., extruded, under a very high force through a channel that extends longitudinally into a die and has a constant cross-section and a bend. The angle ("PHgr") of the bend may range from a few degrees of curvature to approximately 90 degrees. For further information on ECA pressing, see, e.g., S. D. Terhune et al, xe2x80x9cThe Evolution of Microtexture and Grain Boundary Character during ECA Pressing of Pure Aluminumxe2x80x9d, The Fourth International Conference on Recrystallization and Related Phenomena, Edited by T. Sakai et al, The Japan Institute of Metals, 1999, pages 515-522; and Z. Horita et al, xe2x80x9cEqual-channel Angular Pressing for Grain Refinement of Metallic Materialsxe2x80x9d, The Fourth International Conference on Recrystallization and Related Phenomena, Edited by T. Sakal et al, The Japan Institute of Metals, 1999, pages 301-308. Through the ECA process, the amount of deformation imparted to the specimen obviously increases as the angle of the bend increases to 90 degrees and as a radial distance from an apex (origin) of the arc to a point on the specimen increases. As a result, here, too, the strain imparted to the specimen is highly non-uniform, but with the strain varying across the specimen cross-section. The largest amount of strain, here caused by shear, occurs for the region of the specimen that moves along the largest curvature in the arc, i.e., at the largest radial distance from the apex of the arc. Consequently, to compensate for the non-uniform strain, several pressings are often necessary, with the deformed specimen being rotated typically by 90 degrees between each pressing. Furthermore, owing to the high, but non-uniform strain imparted to the specimen as well as significant friction occurring between the specimen and the channel, the specimen experiences non-uniform heating during each pressing. The heating effects have proven. to be extremely difficult, if at all, to control. Since a crystalline micro-structure is also highly influenced by a temperature, these heating effects further distort the grain structure. Therefore, in view of the very high forces required for this technique and the non-uniform results produced thereby, this technique has proven rather impractical for use in accurately simulating a production environment.
Thus, a need still exists in the art for apparatus for a thermodynamic material testing system, and specifically for such apparatus that can impart, on a practical basis, high strains to a metallic specimen in an amount sufficient to appreciably reduce specimen grain size but in a manner which results in a highly uniform micro-structure, on a cross-sectional basis, throughout the work zone of the specimen. Furthermore, such apparatus should provide accurate control over the temperature of the work zone, by selective and controlled specimen heating and/or cooling, such that a specific schedule of deformation and heat treatments can be imparted to the specimen that collectively and accurately simulate actual warm/cold working production environments. Advantageously, such apparatus could facilitate the development of techniques for massively producing relatively low-cost materials that possess very fine grained micro-structures with significantly increased material strength.
My invention is capable of imparting high strain uniformly throughout a work zone of such a specimen thus producing a highly uniform, very fine microstructure throughout that zone in a manner that advantageously overcomes the deficiencies associated with conventional material testing systems.
In accordance with my inventive teachings, the inventive apparatus prevents longitudinal flow elongation, that otherwise results in conventional testing systems, when a work zone of a specimen is highly strained, e.g., compressed between two opposing compression anvils, but permits specimen material flow to occur outward, i.e., sideways, from the work zone. Further, the specimen is rotated between successive compressions through an predefined angle, such as 90 degrees, in order to present strained specimen material to the opposing anvil faces for compression during the next hit.
By virtue of this sideways material flow in the work zone, the specimen is simply rotated between successive hits and then the same strained material is compressed again. This process can simply be repeated multiple times, thereby advantageously inducing very high cumulative strains in the work zone. As a result of increasing strain, increasingly fine grain sizes can be produced in the work zone until the cumulative strain causes work zone material to re-crystallize or the work zone to lose its integrity. As such, the amount of strain that can be induced in the work zone is not limited by the apparatus but rather by the work zone material itself.
Such cumulative strains are considerably greater than those obtainable in practice through conventional material testing systems; hence, yielding smaller grain sizes than heretofore possible with those systems.
While the work zone will bulge outward somewhat as a result of each compressive deformation, essentially the same amount of material, with only some slight change, will remain in the work zone after each hit. By repeatedly deforming specimen material that has flowed sideways in the work zone while constraining longitudinal specimen flow elongation, adequately-sized material remains in the work zone itself to readily support subsequent machining and analysis and/or further testing of the specimen. In addition, since the specimen ends themselves, as gripped in the apparatus, do not change size from flow elongation, the resulting specimen can be readily held during such machining without any need to attach separate ends to the specimen. Hence, the inventive technique eliminates any adverse change in the crystalline micro-structure that could otherwise occur through welding or otherwise attaching such ends to the resulting specimen.
Specifically, in accordance with my particular inventive teachings, the specimen work zone is situated between two opposing compression anvils, each of which is movable with respect to the other. The specimen is securely held in a grip assembly in Which the specimen is fixedly restrained between two grips, each of which grips a corresponding end of the specimen, and oriented such that a compression axis of the specimen lies transverse to the longitudinal axis of the specimen. The grip assembly rigidly holds the specimen during each compressive deformation with sufficient force to prevent the specimen from flow elongating as a result of each compression. Furthermore, the grip assembly is mechanically coupled to a torque motor which rotates the grip assembly throughout a predefined partial angular rotation, typically 90 degrees, between successive compressive deformations. As such, the same strained work zone material is successively presented to opposing faces of the compression anvils and then compressed again durin g each such deformation. The velocity at which the anvils move and the distance through which each anvil moves are selectively controlled through a corresponding servo-controlled hydraulic actuator that drives each anvil in order to set a desired strain rate and final strain attainable through each deformation.
In accordance with a feature of my invention, the inventive apparatus also has the capability to pass controlled amounts of alternating (AC) electric current (at power line frequencies) lengthwise through the specimen before, during and/or after each deformation and also, through water quenching internal to the ends of the specimen, to conductively cool the specimen ends from an elevated temperature. This current causes the specimen to self-resistively heat and establish isothermal planes at a desired substantially uniform temperature throughout the work zone of the specimen. By controlling the rates at which the specimen work zone self-resistively heats and then conductively cools, the work zone can be dynamically set to experience any one of a wide range of different time dependent temperature profiles with relatively little, if any, thermal gradients appearing throughout the work zone. Through accurate control of both specimen deformation and work zone temperature, the specimen can undergo not only substantially the same mechanical deformation but also substantially the same thermal processing that will be encountered in a modern medium to high speed multi-stand rolling mill.
Consequently, the inventive apparatus can be used to very accurately simulate such a mill, and in particular to determine proper mill parameters necessary to produce a very fine grained crystalline micro-structure in a metallic material sufficient to impart relatively high strength to that material.